1. Field of the Invention
The present invention relates to semiconductor laser devices, and more particularly, to a semiconductor laser device capable of reducing noises due to optical feedback of output laser light and a method of designing the same.
2. Description of the Prior Art
In a conventional semiconductor laser device, if optical feedback of output laser light is incident again on a semiconductor laser device, noises due to the optical feedback (hereinafter referred to as optical feedback noises) are generated within the output laser light. If the semiconductor laser device is used as a light source of an optical disk system, for example, optical feedback of output laser light by reflection from a disk surface or the like is incident again on the semiconductor laser device, thereby to generate such optical feedback noises.
In order to reduce the optical feedback noises of the semiconductor laser device, a method utilizing self-sustained pulsation phenomena has been known. For example, a self-sustained pulsation type semiconductor laser device comprising a layer having approximately the same composition as that of an active layer and having saturable light absorbing characteristics with respect to lasing light (a saturable light absorbing layer) is disclosed in Japanese Patent Laying-Open No. 84891/1986 (H01S 3/18), for example. In addition, a semiconductor laser device in which a saturable light absorbing layer is laminated on an active layer excluding a stripe-shaped region in its center is disclosed in Japanese Patent Laying-Open No. 171186/1986 (H01S 3/18). A lasing spectrum causes self-sustained pulsation by the saturable light absorbing layer, whereby the coherence is lowered, thereby to reduce optical feedback noises.
A semiconductor laser device used as a light source of an optical disk system requires low-noise characteristics at the time of reproduction, while requiring high-power characteristics at the time of recording. In a general optical disk system, light output power of 3 mW is required at the time of reproduction, while light output power of 30 mW is required at the time of recording.
However, the semiconductor laser device may not stably carry out the self-sustained pulsation only by the above described construction. Further, in the above described self-sustained pulsation type semiconductor laser device, a kink level (a light output power level at which a kink appears in light output power-current characteristics) is lowered by the saturable light absorbing layer introduced so as to generate the self-sustained pulsation, thereby to make it impossible to select both self-sustained pulsation and high-power lasing.
In the semiconductor laser device having the saturable light absorbing layer, the lower the reflectivity on a light output facet is, the more advantageously higher power is realized. In the conventional self-sustained pulsation type high-power semiconductor laser device, therefore, the reflectivity on a light output facet is set to not more than 5%. In the conventional self-sustained pulsation type high-power semiconductor laser device, however, optical feedback noise characteristics are liable to be degraded when the optical feedback ratio (the ratio of the light quantity of optical feedback to the light quantity of output laser light) is in the-vicinity of 0.1%.